Nearly $4 billion will have been spent on this year’s midterm elections, a staggering figure that makes them the most expensive midterm elections in U.S. history, and $333 million more than the 2010 midterms. The Center for Responsive Politics (CRP) estimates the candidates and parties will have spent around $2.7 billion, while outside groups funding candidates will likely spend $900 million, a total of $3.67 billion.

Republican candidates and right-leaning outside groups will have spent more money than Democrats and liberal-leaning groups, at $1.92 billion for the GOP compared to the Democrats’ $1.76 billion, CRP predicts. But despite the billions spent to woo voters, midterms have a historically low voter turnout compared to presidential elections. The Pew Research Center predicts it will be no different this year, with less than half of eligible voters likely to get to the polls.

The 2010 Citizens United ruling, the Supreme Court decision that paved the way for corporations to donate unrestricted funding to candidates, has had a profound effect on the huge sums spent on elections and is "directly traceable" to the spike in spending, Lawrence Norden, a deputy director for New York University's Brennan Center for Justice, told ABC News. Both Super PACS and more elusive "dark money" groups, which are classified as non-profit organizations and don't have to send their donor lists to the Federal Election Commission (FEC), have benefited from Citizens United. This year, dark money groups have spent $200 million in 11 of the most competitive Senate races, nearly double the amount they forked out in 2012, ABC News reports.